Long-Term Monitoring of a Campus-Scale Geothermal Heat Pump System Using Distributed Temperature Sensing

Geothermal heat pumps are a fast-growing technology for decarbonizing heating and cooling systems. Monitoring existing systems over time can provide useful information for how subsurface heterogeneities will impact efficiency in new systems. Carleton College (Northfield, Minnesota) installed a district-scale, closed-loop geothermal heat pump system in 2018, with over 200 vertical boreholes that extend ~150 m through Paleozoic sedimentary rocks. Fiber-optic distributed temperature-sensing cables in five boreholes provide insight into daily, seasonal, and yearly temperature patterns. At certain intervals, downhole temperature consistently deviates toward the pre-operational temperature, cre-ating persistent pinch points across times and in all seasons, which we attribute to higher groundwater flow. Vertical wells in two borefields are grouted differently: Wells in one field only have thermal grout, while wells in the other field have an interval of pea gravel through an interval of predicted high groundwater flow. Total energy exchange data for each borefield suggest that the field with pea gravel is more efficient than the fully grouted one. There are hints that the subsurface reservoir acts like a sink (absorbing and transmitting thermal energy so that the temperature oscillates around a constant mean), rather than like a battery (storing thermal energy in summer to be released in winter), but more data are needed to fully characterize long-term patterns. The influence of groundwater flow and the long-term responses of the subsurface have implications for borefield design and highlight the importance of monitoring new geothermal systems.